Glass is used almost exclusively as a covering for rigid photovoltaic modules. Coverings made of glass are characterised by low mechanical strength.
Coverings of transparent plastics instead of glass have been disclosed. They consist mainly of polycarbonate sheets. They are used when a higher mechanical strength is required as is the case for example on sailing ships. Here the modules are used in the deck structures so that it cannot be excluded, for instance, that the modules may be trodden on. Modules with polycarbonate as a covering have the disadvantage that they are not very weather resistant. In addition they are permeable to water vapour so the photovoltaic modules can corrode. This means that these modules have to be replaced after a relatively short time.
The use of fluorinated polymers as transparent covering films in lightweight, flexible photovoltaic modules has already been disclosed. Here, either pure fluorinated polymers such as e.g. polyvinyl fluoride (PVF), or modified fluorinated polymers such as ethylene/tetrafluoroethylene copolymers (ETFE), are used. Tedlar.RTM. or Tefzel.RTM., both commercial products from the DuPont Co., are used as films for the purpose mentioned above.
Three-layered films with the sequential structure polyvinyl fluoride/polyester/polyvinyl fluoride are also used as backing films for photovoltaic modules. An example of these is Icosolar.RTM., a commercial product from the Isovolta Co.
Photovoltaic modules with coverings made from fluorinated polymer films have low mechanical strength. Also, it is difficult to print on fluorinated polymer films.